It is for the 400 turbo. It drops into passing gear(automatic down shift) from an electrical solenoid on the side of the trans. Unlike the 350 turbo, that uses a cable that runs from the carb to inside the trans next to the cooler lines.

So that is the switch to run the solenoid at close to full throttle. I am sure they put it inside the car, so it would last longer then being in the elements